A related art fuel cell system is discussed below with reference to FIG. 5.
The related art fuel cell system includes a fuel cell 1 that generate electric power from a fuel gas and an oxidant gas, a fuel generator 2 that adds water (W) to a feedstock (Fs), such as natural gas, to reform the feedstock material and thereby produce a hydrogen-rich fuel gas, a blower 3 that supplies an oxidant gas (O), such as air, to the fuel cell 1, a fuel supplying conduit 4 that supplies the fuel gas produced by the fuel generator 2 to the fuel cell 1, and a fuel exhausting conduit 5 that discharges a remaining fuel gas, which has not been consumed by the fuel cell 1.
The related art fuel cell system further comprises a first bypass conduit 6 that does not feed the fuel gas to the fuel cell 1 but makes the fuel gas flow from the fuel supplying conduit 4 to the fuel exhausting conduit 5, a gas pathway switching device 7 that switches over an outflow pathway of the fuel gas between the fuel supplying conduit 4 and the first bypass conduit 6, an on-off valve 8 that prevents the fuel gas from flowing reversely or diffusing from the fuel exhausting conduit 5 to the fuel cell 1 during a feed of the fuel gas to the first bypass conduit 6, a combustion chamber 9 that burns the natural gas, the fuel gas, or the remaining fuel gas and raises the temperature in the fuel generator 2, and a filter 10 that traps undesired substances, such as a deteriorated catalyst accommodated in the fuel generator 2, to prevent the flow of such substances into the fuel cell 1.
On security grounds, while the fuel cell system does not work, the combustible gases are to be expelled from the gas pathways in the fuel cell system. Before and after operation of the fuel cell system, a nitrogen gas (N) is flown through a feedstock supplying pathway to the fuel generator 2, is subsequently circulated from the fuel generator 2, through the gas pathway switching device 7, the fuel supplying conduit 4, the first bypass conduit 6, the fuel cell 1, the fuel exhausting conduit 5, and the on-off valve 8, and is discharged from the combustion chamber 9.
At a start of the operation of the fuel cell system, the supply of nitrogen is cut off, and the feedstock is fed to the fuel generator 2. Before the fuel generator 2 reaches a preset temperature (about 700° C.), at which the hydrogen-rich fuel gas is generated, the feedstock is introduced by the gas pathway switching device 7 from the fuel generator 2 to the first bypass conduit 6, led to the combustion chamber 9 and subjected to combustion in the combustion chamber 9. When the temperature in the fuel generator 2 is raised to the preset level of approximately 700° C. by combustion of the feedstock in the combustion chamber 9, water is fed to the fuel generator 2, which then starts generation of the fuel gas.
The fuel generator 2 has a carbon monoxide removing unit that reduces the concentration of carbon monoxide contained in the fuel gas to a specific level that does not damage a catalyst in the fuel cell 1. The temperature in the carbon monoxide removing unit is varying for several tens of minutes since a start of generation of the fuel gas. The resulting fuel gas accordingly has a relatively high concentration of carbon monoxide. The fuel gas of the high carbon monoxide content is not fed to the fuel cell 1, but is introduced by the gas pathway switching device 7 to the first bypass conduit 6, led to the combustion chamber 9 and subjected to combustion in the combustion chamber 9. The on-off valve 8 is set closed to prevent the fuel gas of the high carbon monoxide content from flowing reversely from the fuel exhausting conduit 5 to the fuel cell 1.
When the temperature in the carbon monoxide removing unit of the fuel generator 2 is stabilized and the concentration of carbon monoxide contained in the fuel gas becomes not greater than 20 ppm, the on-off valve 8 is set open. The fuel gas is introduced by the gas pathway switching device 7 to the fuel supplying conduit 4 and led to the fuel cell 1. The blower 3 is simultaneously actuated to feed the oxidant gas to the fuel cell 1, which then starts power generation.
The temperature in the fuel generator 2 is kept at the preset level of approximately 700° C. by controlling the flow rate of the feedstock fed to the fuel generator 2 and thereby adjusting the quantity of the fuel gas introduced into the combustion chamber 9. In the case of the lower temperature of the fuel generator 2, the quantity of supply of the feedstock is increased to increase the fuel gas produced by the fuel generator 2 and thereby enhance the quantity of the fuel gas introduced into the combustion chamber 9. In the case of the higher temperature of the fuel generator 2, on the other hand, the quantity of supply of the feedstock is decreased to decrease the fuel gas produced by the fuel generator 2 and thereby reduce the quantity of the fuel gas introduced into the combustion chamber 9.
In the related art fuel cell system, however, the fuel cell 1 is hermetically sealed until the temperature of the carbon monoxide removing unit of the fuel generator 2 is stabilized after the start of the operation. The pressure in the fuel cell 1 may thus rise to a damaging level causing breakage of the fuel cell 1.
The fuel gas is discharged from the fuel generator 2 typically at temperatures of approximately 100° C. The gas pathway switching device 7 is accordingly required to have a heat resistance to the temperatures of about 100° C. in the related art fuel cell system. Construction of the gas pathway switching device 7 using a high heat-resistant valve, however, undesirably raises the equipment cost.
For adjustment of the temperature in the fuel generator 2, it is required to control the flow rate of the feedstock. The variation in flow rate of the feedstock results in varying the flow rate of the fuel gas supplied to the fuel cell 1 and thus does not ensure stable power generation by the fuel cell 1.
Accumulation of undesired substances on the filter 10 prevents the smooth supply of the fuel gas to the fuel cell 1. The filter 10 is thus required to have a replaceable structure. This undesirably complicates the construction of the fuel cell system, raises the equipment cost, and requires occasional maintenance. Accumulation of a large quantity of undesired substances on the filter 10 within a short time period makes the fuel cell system inoperative.
In the related art fuel cell system, circulation of nitrogen through the gas pathway before and after the operation of the system. This requires a nitrogen tank and thus undesirably raises the equipment cost. The fuel cell system can not be driven when the nitrogen tank does not contain a sufficient amount of nitrogen.